When mild steel is cut with a laser having a small power relative to the thickness of the mild steel, sufficient cutting laser energy can be obtained at the vicinity of the surface of the mild steel at which the laser is applied. However, there are many cases in which sufficient cutting laser energy cannot arrive at the inside of the mild steel. Therefore, in recent years, high concentration (purity) oxygen (having a concentration (purity) of 99.5% or more) is supplied to the vicinity of the cutting point by use a co-axial double nozzle 1a and 1b as shown in FIGS. 10 and 11, a co-axial triple nozzle (not shown in Figures), or an auxiliary nozzle 2 as shown in FIG. 12. Thereby, a sufficient oxygen concentration for cutting the inside of the mild steel is maintained. Heat is generated due to the oxidation reaction between steel and the supplied oxygen, and the heat compensates for the deficiency of laser energy of the laser cutter.
In the laser cutting method using the co-axial double nozzle 1 or the auxiliary nozzle 2, only the oxygen quantity is adjusted, and the oxygen concentration is not adjusted. Therefore, it is difficult to maintain a suitable cutting quality when the thickness or the kinds of the material to be cut is changed. An excess combustion heat is generated due to the small change in conditions when the material is cut, and easily adversely effects to a quality of the cut surface.
That is, when the mild steel is cut at a high speed (1.5 m/min or greater), a high temperature area 3 (represented by a two-dot dashed line in FIGS. 13 and 14) suitable for combustion between oxygen and steel is generated at the cutting surface behind the cutting point in a cutting direction, rather than the forward to the cutting point in a cutting direction by the influence of the heat conductivity and the thermal diffusivity, as shown in FIGS. 13 and 14. Then excess combustion energy adversely effects to the cutting surface and the cutting quality. Consequently, for example, as shown in FIG. 15, a normal cutting quality can be obtained at the vicinity of the surface 4 of the material to be cut; however, an abnormal cutting quality is often obtained on the inside of the material to be cut. In addition, a self burning phenomenon, dross adhesion, or cutting notch generation easily occurs on the inside of the material. Therefore, it is impossible to sufficiently improve the cutting quality. When the laser beam pulse is output continuously in order to cut the material at a high speed, the frequency of occurrence of the self burning phenomenon increases. Therefore, in order to prevent the occurrence of the self burning phenomenon, usually the laser beam pulse is intermittently output. However, when the laser beam pulse is intermittently output, the cutting speed decreases.
When the laser cutter applying a laser beam 5 and a cutting gas (high concentration oxygen) 6 and 6 from the same nozzle shown in FIGS. 13 and 14 is used, the cutting gas 6 and 6 diffuses more widely than the diameter of the laser beam 5. The gas stream of the cutting gas 6 consequently generates behind the cutting point in a cutting direction. When the diameter of the nozzle is set so as to equal the diameter of the laser beam, the combustion energy at the high temperature distributional area formed behind the cutting point in the cutting direction, increases due to oxygen. Therefore, it is impossible to solve the problem. The cutting quality depends on the position of the center of the laser beam 5. In order to adjust the proper diameter of the nozzle against laser beam diameter 5, it is necessary to frequently change the nozzle.